[InstrProf] Add option to avoid instrumenting small functions

If a function only has a few instructions, instrumentation can significantly increase the size and performance overhead of that function. Add the `-pgo-function-size-threshold` option to select a size threshold so these small functions are not instrumented.

A similar option `-fxray-instruction-threshold=<N>` is used for XRay to reduce binary size overhead [1].

[1] https://www.llvm.org/docs/XRay.html

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D131816